As a recent trend in cutting tools, the cutting edge temperature of the tools has been becoming increasingly higher for reasons including the demand for dry machining without the use of a cutting lubricant from the viewpoint of global environmental conservation, diversified workpieces, and increased cutting speeds for higher machining efficiency. Accordingly, the properties required of tool materials have been becoming stricter. As the properties required of tool materials, particularly, not only the heat resistance of a coating formed on a substrate, but also the improvement in wear resistance and the lubrication performance of the coating to replace a lubricant, which are related to the lives of cutting tools, have been becoming more important.
For improvements in the heat dissipation, lubricity, and chipping resistance of the coating, a technique is well known that forms a coating of AlN on the surface of a cutting tool formed of a hard substrate such as a WC-based cemented carbide, a cermet, or a high-speed steel. AlN, which has high thermal conductivity, can improve the heat dissipation of the coating and does not trap heat in itself In addition, AlN features high lubricity with low hardness. This feature gives AlN the advantage of preventing abnormal tool wear and improving the chipping resistance.
Having such various advantages, AlN is almost an essential material for achieving a balance between the lubricity and chipping resistance of cutting tools at a high level. Accordingly, AlN has been used in various ways. PTL 1, for example, discloses a technique that uses AlN in a hexagonal crystal state for the outermost surface. PTL 2 discloses a technique that forms a compound layer containing aluminum and one or more elements selected from the group consisting of nitrogen, oxygen, and carbon by physical vapor deposition. Similarly, PTL 3 discloses a technique that uses AlN for the surface of a coating. Thus, a coating of AlN can be formed on the outermost surface to improve the heat dissipation, lubricity, and chipping resistance of that surface.
However, all of the coatings of AlN disclosed in PTL 1 to 3 cause a heat crack in the tool substrate because they quickly transfer heat generated during cutting to the tool substrate (through a lower layer if any) due to the high thermal effusivity of AlN. This results in the problem of a shortened tool life. In addition, all of the coatings of AlN disclosed in PTL 1 to 3 have an insufficient lubrication effect because they wear quickly due to their insufficient hardness.
As an attempt to further improve the lubricity, on the other hand, PTL 4 discloses a technique that adds chlorine to an outermost coating of AlN to improve the lubricity of the outermost surface of the coating. In addition, PTL 5 discloses a technique that improves the thermal insulation and lubricity of the surface of the coating by forming TiCN and TiCNO layers on the substrate side of the coating and forming an Al2O3 layer, which has high heat resistance, and an AlN layer, which has high lubricity, on the outermost side of the coating.